Problems often found in a cellulose ether having a low degree of polymerization are yellowing and discoloration. Such yellowing or discoloration is also found in a composition containing the cellulose ether. Yellowing or discoloration can easily be observed in transparent pharmaceutical capsules and white food compositions.
There has been a demand for a method for preparing a cellulose ether having a low degree of polymerization, the cellulose ether having a low degree of yellowing or discoloration or free from yellowing or discoloration. There has also been a demand for pharmaceutical capsules and food using such a cellulose ether having a low degree of polymerization.
As the methods of decreasing the degree of polymerization of a cellulose ether, there has been proposed, as well as a method of depolymerizing a cellulose ether with hydrochloric acid as shown in the Japanese Phase Publication No. 2002-531594 of WO 00/32637, the Japanese Phase Publication No. 2003-503557 of WO 01/000680, or JP 2005-239845A, a method for preparing a cellulose ether comprising a step of bringing cellulose and an alkali metal hydroxide solution into contact with each other to obtain a corresponding alkali cellulose and then reacting the resulting alkali cellulose with oxygen for depolymerization as shown in JP 2001-002701A.